1. Field of the Invention
The present invention relates to a recordable optical information recording medium where information can be recorded and reproduced with laser beams having a predetermined wavelength.
2. Description of the Related Art
Conventionally, an optical information recording medium (optical disk) capable of recording information only once with laser beams is known. This optical disk is also called a CD-recordable (CD-R) and typically has, on a transparent disk substrate, a recording layer made of an organic dye, a reflective layer made of a metal such as gold, and a protective layer (cover layer) made of a resin. The CD-R is irradiated with near-infrared laser beams (usually laser beams having a wavelength in the vicinity of 780 nm), the irradiated region of the recording layer absorbs the beams, and the temperature thereof increases, whereby physical or chemical changes occur (e.g., forming pits) to change the optical characteristics in that region, and thereby information is recorded on the CD-R. This information is read (reproduced) by irradiating the CD-R with a laser beam having a wavelength identical to those of the laser beams used in recording, and detecting a difference in reflectance between the region of the recording layer having changed optical characteristics (recording region) and the unchanged region (unrecorded region). This CD-R is advantageous in that it can be played back on commercial CD players, and in recent years its demand is increasing with the spread of personal computers.
Additionally, there is a demand for an optical recording medium having higher recording density. In response to this demand, an optical disk called a digital versatile disk-recordable (DVD-R) was proposed (for example, see “Nikkei New Media”, Extra Volume “DVD”, published in 1995). This DVD-R is structured such that usually, two disks each having a recording layer containing an organic dye, a reflective layer and a protective layer laminated in this order on a transparent disk substrate having a guide groove (pre-groove) for the tracking of laser beam whose width (0.74 to 0.8 μm) is no more than half of the groove width of CD-R are stuck, with the recording layer inside, or the above-described substrate disk and a protective substrate disk having a shape identical with the disk are stuck, with the recording layer inside. Information is recorded on the DVD-R and reproduced from the DVD-R by irradiating visible laser beams (usually, laser beams having a wavelength in the range of 630 to 680 nm) and the DVD-R enables higher density recording than CD-R.
In recent years, networks such as the Internet, as well as high-vision TVs, are rapidly increasing in use and popularity. Further, High Definition Television (HDTV) broadcasting has already been initiated. Considering these circumstances, there is a need for a high-capacity recording medium capable of recording image information inexpensively and easily. DVD-R fulfils this need satisfactorily, but there is an increasing demand for even higher capacity and higher density, and a recording medium capable of dealing with this demand needs to be developed. Accordingly, a high capacity recording medium capable of high-density recording with light of a shorter wavelength than that used for DVD-Rs is under development. In particular, the recordable optical information recording media capable of recording information only once are used more and more frequently for backup or long-term storage of large-capacity information, and thus there is a strong demand for further development.
An optical information recording medium intended to be used in recording using light passed through a lens having a high NA suitable for high-density recording has been disclosed. This optical information recording medium has, on a substrate, in the following order, a reflective layer, a recording layer and a cover layer thinner than the substrate. Additionally, a method of recording and reproducing information is disclosed, wherein laser beams are irradiated onto the recording medium from the cover layer to the reflective layer. The laser beams have a short wavelength of no more than 550 nm. As the dye of the recording layer of such an optical information recording medium for use with laser beams having the above-described short wavelength, porphyrin compounds, azo dyes, metal azo dyes, quinophthalone dyes, trimethine cyanine dyes, dicyanovinyl phenyl skeleton dyes and coumarin compounds have been proposed (e.g., Japanese Patent Application Laid-Open (JP-A) Nos. 4-74690, 7-304256, 7-304257, 8-1271705, 11-53758, 11-334204, 11-334205, 11-334206, 11-334207, 2000-43423, 2000-108513, 2000-149320, 2000-158818, 2000-228028 and 2001-146074). It is easy for those skilled in the art to anticipate that compounds other than those described above can also be used in principle as the dye of the recording layer, insofar as the compounds can absorb laser beams used in recording. Proposed colors for recording and reproduction laser beams include blue-violet (wavelength of 405 or 410 nm), blue (wavelength of 430 or 488 nm) or blue-green (wavelength of 515 nm).
Generally, optical information recording media capable of recording and reproducing information using laser beams having a short wavelength have, for example, a reflective layer, a recording layer, a barrier layer, a tackifier or adhesive layer and a cover layer provided on a substrate in this order, which is different in layer structure from that of a CD-R and a DVD-R. Accordingly, the laser irradiation direction of this recording medium is opposite to that of a CD-R and a DVD-R, and layer peeling not problematic in CD-Rs or DVD-Rs may be problematic in this recording medium. Accordingly, strong adhesion between the respective layers is required. However, the layers are constituted such that an organic material (the recording layer and the like) is adjacent to inorganic materials (the reflective layer, the barrier layer, and the like), thus generating easily peelable interfaces. Therefore, sufficient adhesion cannot be obtained in certain cases and there is a need for further improvement. Due to this layer structure, the recording layer, particularly the outer peripheral edge portion thereof, is exposed to the outside, and thus shelf life may deteriorate due to atmospheric temperature and humidity.
In the above-described optical information recording medium used in an optical recording system utilizing a blue-violet laser and high NA pickup, the cover layer through which the laser beams pass is preferably thin so as to adjust the focus of an object lens having high NA. A thin film is used as the cover layer and bonded via an adhesive or tackifier onto the entire surface of the recording layer. The thickness of the cover layer including an adhesive or tackifier layer formed by curing the adhesive or tackifier is usually about 10 μm, but varies according to the wavelength of the irradiated laser or NA,
The cover layer constitutes a surface through which light passes (light incidence surface) as described above, so that if the cover layer cannot adhere uniformly to the recording layer, swelling or warping appears on the surface, which lowers the reflectance due to scattering of incident light. This results in failure to achieve satisfactory recording characteristics.
Therefore, there is a need for an optical information recording medium having strong adhesion between layers, excellent shelf life and stable recording and reproduction characteristics.
Also, there is a need for an optical information recording medium including a cover layer which has high surface uniformity (smoothness) of a light incidence surface and having excellent recording characteristics.